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Heterogeneous Accretion and the Moderately Volatile Element Budget of Earth

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Science  14 May 2010:
Vol. 328, Issue 5980, pp. 884-887
DOI: 10.1126/science.1186239

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Earth's Silver Lining

The age of the oldest rocks on Earth's surface is controversial, but, even if they are at their oldest estimate, hundreds of millions of years in our planet's earliest history are still missing. However, in some rocks that until relatively recently resided in the mantle, the isotopic signature from the time of Earth's formation is still preserved. Schönbächler et al. (p. 884) exploited this preservation to constrain models that describe the early material that assembled together to form Earth. Because the isotopic profile of silver in these rocks is nearly identical to that measured in a class of primitive meteorites, the earliest material probably had high volatile content. However, the fractionation of other isotopes suggests that the volatile content probably decreased over time in subsequent accretion events. With these isotopic model constraints, it is possible that one of the last major collisions—the Moon-forming giant impact—added considerable amounts of water and other volatile elements to Earth.

Abstract

Several models exist to describe the growth and evolution of Earth; however, variables such as the type of precursor materials, extent of mixing, and material loss during accretion are poorly constrained. High-precision palladium-silver isotope data show that Earth’s mantle is similar in 107Ag/109Ag to primitive, volatile-rich chondrites, suggesting that Earth accreted a considerable amount of material with high contents of moderately volatile elements. Contradictory evidence from terrestrial chromium and strontium isotope data are reconciled by heterogeneous accretion, which includes a transition from dominantly volatile-depleted to volatile-rich materials with possibly high water contents. The Moon-forming giant impact probably involved the collision with a Mars-like protoplanet that had an oxidized mantle, enriched in moderately volatile elements.

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